Detecting the zero-crossing points of Alternating Current (AC) waveforms is useful in a variety of applications, such as AC motor control, microprocessor-based dimmers and Power-Line Communication (PLC). Various techniques and circuit designs for zero-crossing detection are known in the art.
For example, U.S. Patent Application Publication 2008/0157822 describes a zero-crossing point detection circuit that includes a hot line input, a neutral line input, a first zero-crossing point output and a first optical coupler. The first optical coupler includes a first light-emitting diode (LED) and a first optical transistor. The hot line input and neutral line input are respectively connected to two terminals of the first LED. An emitter of the first optical transistor is grounded. A collector of the first optical transistor is connected to a direct current (DC) power source. The collector of the first optical transistor is also connected to the first zero-crossing point output.
As another example, U.S. Patent Application Publication 2011/0279163 describes a signal level crossing detector circuit that includes a DC isolator and a detector circuit. The DC isolator has at least a first input, which is operable to receive a high voltage AC signal, and at least a first capacitor, a first plate of the first capacitor being electrically connected to the first input. The detector circuit is operable at a low voltage and has at least a first detector input, the first detector input being electrically connected to a second plate of the first capacitor, the low voltage detector circuit being operable to provide a change in output signal in dependence on a high voltage AC signal on the first input crossing a predetermined signal level. The signal level crossing detector may be single ended or differential.
U.S. Patent Application Publication 2008/0309379 describes a zero crossing circuit that includes a signal output circuit element for registering a sharply defined signal, and in one embodiment an isolation circuit element cooperating with the signal output element, and a delay-inducing circuit element cooperating with the signal output element for applying a substantially constant time delay to the signal. The delay-inducing element includes a switch circuit and a delay circuit. The switch circuit commences the time delay by the delay circuit upon a triggering voltage being reached. The time delay circuit is adapted so that the time delay equates to a time period required for the triggering voltage to change to zero so as to cross zero voltage substantially as the time delay expires.
The description above is presented as a general overview of related art in this field and should not be construed as an admission that any of the information it contains constitutes prior art against the present patent application.